FIG. 1 shows a conventional photovoltaic cell 1. Photovoltaic cell 1 comprises a planar semiconductor material 3. Material 3, generally made of polysilicon, comprises three areas of different doping. A thick central area 3a is lightly P-type doped. An upper area 3b is N-type doped, and possibly overdoped at its surface. A lower area 3c is heavily P-type doped (P+). A conductive comb 5 is placed above area 3b, intended to be exposed to light. An aluminum layer 6 covers the lower surface of the cell. Comb 5 and layer 6 are both intended to transmit the photovoltaic current and are connected to the + and − terminals, not shown, of the cell. An antireflection layer, not shown, is preferably placed on area 3b and comb 5 to limit the reflection of light rays at the photocell surface.
Material 3 conventionally originates from a polysilicon bar obtained from a silicon melt. The bar is sawn to obtain wafers which are then doped to obtain material 3. This manufacturing method, close to the single-crystal silicon wafer manufacturing method, is expensive and limits the possible wafer dimensions.
The inventor has disclosed in a conference in Munich (17th European Photovoltaic Solar Energy Conference and Exhibition, Munich, 21-26 Oct. 2001) a method of manufacturing polysilicon wafers by sintering of silicon powders. In this method, silicon powders of 5 μm or 20 μm are placed between the plates of a press. The assembly is compressed with a pressure P ranging between 70 MPa bars) and 900 MPa (9,000 bars). Then, the compacted layer is introduced into a sintering furnace, where it is heated up to a temperature T ranging between 950° C. and 1050° C. The sintering, which enables growth of bridges between the grains and stiffening of the material, has been performed at the indicated temperatures for a time range from two to eight hours, under a low argon pressure (100 Pa).
The obtained materials have sufficient mechanical strength to be able to be handled. However, their porosity is high, above 15%. Further, the grain size is small, since the size of said grains has not substantially increased during the processing. The mobility-lifetime product of the minority carriers is low, on the order of 10−7 cm2V−1 (10−11 m2V−1 in the international system). The obtained materials are unusable in the photovoltaic field. For example, due to the high porosity, it is impossible to dope a specific area of the material, because of the dopants migrating through the porosity channels and spreading everywhere inside the material. As to the mobility-lifetime product of the minority carriers, values at least one thousand times greater are required for the material to be used in a photocell. Further, the surface of the obtained materials is uncontrolled and rough. Such a surface state prevents the provision of surface junctions, necessarily poor, in particular, because of significant leakage currents.